Data storage devices are available in varying levels of quality and performance. Typically, the level of quality and performance is inversely proportional to the cost of the storage device. Because the importance of being able to quickly access data, the frequency of data access, and the level of reliability required varies widely among different users and different types of stored data, not all data needs to be stored in high performance storage devices. Also a vast majority of stored data is usually never accessed or accessed only infrequently. Thus, in order to minimize the costs associated with data storage, each piece of data should be stored in a storage device having an appropriate quality and performance level in accordance with the attributes of the data being stored and the needs of the users of the data.
Another issue that storage administrators frequently encounter is low utilization of existing storage resources. Due to recent explosive growth in the amount of data being stored, many administrators do not have enough human resources to closely manage the entire storage system. As a result, it is nearly impossible for administrators to manually manage the expansion of every volume in the system on a day-to-day basis. This lack of manpower to closely manage each volume causes many volumes to be over-allocated so as to be prepared for the possible addition of a large amount of data, which may in fact never occur. This over-allocation ties up huge amounts of unutilized space in conventional storage systems.
Tiered storage is a solution for reducing the cost of storing data by differentiating various types of data, and then storing the data in storage devices that are selected to provide an appropriate level of reliability and performance. For example, a SAN (Storage Area Network) may include plural storage tiers such as a high reliability, high performance, and premium cost first tier that may be used for important data that is accessed often, and a lower reliability, lower performance, and less expensive second tier that may be used for archive data or other infrequently-accessed data. Data can be stored according to a classified type, owner, or the like, and also may be migrated between tiers based on various situations and contingencies. Thus, by using these various levels of tiered storage resources, the total cost of storage can be reduced, while required access speed or reliability for specific data can still be maintained.
Furthermore, thin provisioning is a solution that helps improve the efficiency of storage utilization and eliminate wasted capacity. Thin provisioning systems typically present a virtualized full-volume capacity to a host computer. However, the system actually only allocates real storage capacity to particular portions the thin provisioned volume when the particular portion of the volume receives data to be stored. The units of partial storage capacity allocated to a thin provisioned volume may be referred to as “chunks”, and the chunks of storage may be carved from a storage extent referred to as a “thin provisioning pool” at the time that a chunk is requested. Plural thin provisioned virtual volumes can share a thin provisioning chunk pool. By this arrangement, free space can be flexibly managed, which can reduce the amount of unutilized space in the storage system.
Each of the solutions discussed above can individually help achieve cost reduction and improve system utilization, but attempts to use these two solutions in the same storage system have not been successful in producing the desired increases in efficiency of capacity utilization. Since typically every tier maintains at least one respective thin provisioning pool which is created from storage resources of that tier, when there are a number N of tiers in the storage system, then a number N of redundant free spaces is produced in the storage system. Accordingly, when these multiple free spaces are not utilized, it adds to the overall cost of the system.
Related art includes US Pat. Appl. Pub. 2004/0162958, to Kano et al., entitled “Automated On-Line Capacity Expansion Method for Storage Device”, filed Feb. 23, 2004; US Pat. Appl. Pub. 2006/0069862, to Kano et al., entitled “Method for Managing Volume Groups Considering Storage Tiers”, filed Sep. 29, 2004; and U.S. patent application Ser. No. 11/605,440, to Atsushi Murase, filed Nov. 29, 2006, the entire disclosures of which are incorporated herein by reference.